A transport apparatus of this type is used for example in film scanners, where the synchronism of the scanned film is especially important, said film representing the tape-type medium in this case. The invention is illustrated hereinafter in connection with the aforementioned application, but is not restricted thereto.
In the case of present-day television broadcasts, the television image is very often composed of film sequences and a recording by means of a television camera or digital video effects. Such composite television images make very high demands on the image stability and consequently on the synchronism of a film scanner which is used to scan the broadcast film material. Even fluctuations which are smaller than a television line have a disturbing effect for the viewer.
It quite generally holds true that the precise measurement of the actual transport speed of the tape-type medium—be it film or a different material—is critical for improving the synchronism.
In order to combat the difficulties caused by image instabilities, DE 19531508 A1 proposes a television film scanner having a device for determining and automatically correcting vertical and horizontal image instabilities during the continuous scanning of a film strip. For this purpose, the known television film scanner uses two groups of linear array sensors and image memories, vertical and horizontal correction signals being determined by scanning the perforation holes. The first group of linear array sensors is arranged transversely with respect to the film running direction and serves to determine vertical correction signals. The dimensions of the first group of linear array sensors correspond approximately to the width of the perforation holes transversely with respect to the film transport direction and approximately to the width of a film image along the film transport direction. During operation of the film scanner, the leading edge of a perforation hole generates a sudden change in brightness for each photoelement, the output signals of which are evaluated for determining a vertical correction signal. In order to derive horizontal correction signals, a second group of photosensors are arranged in the region of the left-hand edge of the film. The linear array sensors of the second group run at right angles to the linear array sensors of the first group. The synchronism of the film is regulated by means of a toothed roller tacho pulse and also the vertical correction signals.